1. Field of the Invention
The present invention relates to a traffic shaping device for absorbing fluctuation of delay of cells in an ATM (Asynchronous Transfer Mode) network.
2. Description of the Related Art
In the ATM network for transmitting packets each having a fixed length and called a cell, cells on virtual channels (VC) are multiplexed and then transferred through a virtual path (VP). Since the multiplexing is performed asynchronously in the ATM transfer system, the so-called cell delay fluctuation occurs. If a cell having an interval which is reduced due to the cell delay fluctuation is generated, a burst traffic occurs on the virtual path and so the utilization efficiency of the network resources is lowered. In order to solve such problem, a traffic shaping device has been proposed.
FIG. 1 is a schematic circuit diagram showing an example of a conventional traffic shaping device. In FIG. 1, the conventional traffic shaping device includes an ATM cell input portion 12 for receiving at least one ATM cell 11. The ATM cell input portion 12 sends the ATM cell input thereto to a cell buffer selector portion 13. The cell buffer selector portion 13 selects one or more of first to N-th cell buffers 14.sub.1 to 14.sub.N on the basis of an identifier attached to the ATM cell 11 input to the ATM cell buffer selector portion 13, where N is an integer larger than 1. A cell buffer is provided for every virtual channel and a cell 15 is stored in the cell buffer selected by the cell buffer selector portion 13. Those of the first to N-th cell buffers 14.sub.1 .about.14.sub.N in which cells 15 are already stored output not-empty signals 16 indicative of filled cell buffers. The not-empty signal 16 is sent to an output cell buffer selector portion 17.
The first to N-th cell buffers 14.sub.1 .about.14.sub.N are provided with a first to N-th shaping counters 18.sub.1 .about.18.sub.N, respectively. The first to N-th shaping counters 18.about.18 function to perform a rate control between the cell buffers 14.sub.1 .about.14.sub.N having different shaping rates and to send output request signals 19 to the output cell buffer selector portion 17, independently, with intervals assigned to the respective shaping counters 18.sub.1 .about.18.sub.N. The output cell buffer selector portion 17 selects at least one of the cell buffers 14 which is to be permitted to send a cell stored therein in a cell period corresponding to the maximum rate of a cell output portion 21 provided on the output side of the respective cell buffers 14.sub.1 .about.14.sub.N. In a case where, in a certain cell period, there is an output request from any shaping counter 18 and there is a cell stored in a cell buffer 14 corresponding to this shaping counter 18, a cell output permit signal 22 is output to the cell buffer 14 corresponding to the cell buffer 14. Further, in a case where, in a certain cell period, there are output requests from a plurality of the shaping counters 18 and there are cells stored in a plurality of cell buffers among the cell buffers 14 corresponding to the respective shaping counters 18, a cell output permit signal 22 is sent to one of the cell buffers 14 having the cells stored therein, which stores a cell of a class having the highest priority, according to a predetermined fixed priority. There may be a case where a cell output permit signal 22 is sent to one of cell buffers according to a simple rotation without using a special priority.
In response to the cell output permit signal 22, the selected one of the first to N-th cell buffers 14.sub.1 .about.14.sub.N sends a cell 23 stored therein to the cell output portion 21. The cell output portion 21 sends the cell 23 to a switch portion or a channel adapter portion which is provided in a subsequent stage although not shown. The number of the first to N-th shaping counters 18.sub.1 .about.18.sub.N is the same as the number of the cell buffers 14.sub.1 .about.14.sub.N.
In the conventional traffic shaping device constructed as mentioned above, the shaping counters 18.sub.1 .about.18.sub.N are provided for the respective cell buffers 14.sub.1 .about.14.sub.N for every traffic class or every output port in an ATM switch. The output request signal 19 is sent by one of these shaping counters 18.sub.1 .about.18.sub.N at a rate assigned to the corresponding one of the cell buffers 14.sub.1 .about.14.sub.N.
In a recent ATM switch, in order to guarantee the quality of various traffics, cell buffers arranged correspondingly to the classes of traffics are arranged to make a traffic control possible for every class. Further, in order to offer services of higher quality, a traffic control for every virtual channel has been studied. In order to control the cell sending rate in such case, the shaping counters 18.sub.1 .about.18.sub.N are prepared correspondingly to the respective virtual channels and, therefore, there is a problem that the number of the shaping counters 18.sub.1 .about.18.sub.N is increased with increase of the number of the cell buffers 14.sub.1 .about.14 .sub.N and the amount of hardware in a traffic shaping device becomes very large.
Therefore, an object of the present invention is to provide a traffic shaping device capable of controlling a cell sending rate by means of a common circuit portion thereof even when a plurality of cell buffers exist.